Winds depend primarily on the local distribution of heat in the air, especially on the great amount of heat constantly present in the equatorial zone, due to the sun being always nearly vertical at noon, and to its being similarly vertical at each tropic once a year, with a longer day, leading to even higher temperatures than at the equator, and producing also that continuous belt of arid lands or deserts which almost encircle the globe in the region of the tropics. Heated air being lighter, the colder air from the temperate zones continually flows towards it, lifting it up and causing it to flow over, as it were, to the north and south. But as the inflow comes from an area of less rapid to one of more rapid rotation, the course of the air is diverted, and produces the north-east and south-east trades; while the overflow from the equator going to an area of less rapid rotation, turns westward and produces the south-west winds so prevalent over the north Atlantic and the north temperate zone generally, and the north-west in the southern hemisphere. It is outside the zone of the equable trade-winds, and in a region a few degrees on each side of the tropics, that destructive hurricanes and typhoons prevail. These are really enormous whirlwinds due to the intensely heated atmosphere over the arid regions already mentioned, causing an inrush of cool air from various directions, thus setting up a rotatory motion which increases in rapidity till equilibrium is restored. The hurricanes of the West Indies and Mauritius, and the typhoons of the Eastern seas, are thus caused. Some of these storms are so violent that no human structures can resist them, while the largest and most vigorous trees are torn to pieces or overturned by them. But if our atmosphere were much denser than it is, its increased weight would give it still greater destructive force; and if to this were added a somewhat greater amount of sun-heat--which might be due either to our greater proximity to the sun or to the sun's greater size or greater heat-intensity, these tempests might be so increased in frequency and violence as to render considerable portions of the earth uninhabitable. The constant and equable trade-winds have a very important function in initiating those far-reaching ocean-currents which are of the greatest importance in equalising temperature. The well known Gulf Stream is to us the most important of these currents, because it plays the chief part in giving us the mild climate we enjoy in common with the whole of Western Europe, a mildness which is felt to a considerable distance within the Arctic Circle; and, in conjunction with the Japan current, which does the same for the whole of the temperate regions of the North Pacific, renders a large portion of the globe better adapted for life than it would be without these beneficial influences.